The present invention relates to a method of storing, erasing and reading binary optical information and, more particularly, to the use of photoactivated shifts in the phase transition between the ferroelectric (FE) and the antiferroelectric (AFE) phases within a polycrystalline PLZT (lead lanthanum zirconate titanate) film containing device to store, erase and read binary information. The U.S. Government has rights in this invention pursuant to Contract No. DE-AC04-76DP00789, awarded by the U.S. Department of Energy.
This application is related to U.S. Pat. application Ser. No. 200,104, filed May 31, 1988, by Cecil Lamb and assigned tot he assignee of this application.
Since the invention of the computer, efficient storage and retrieval of digital information has been a major area of interest. The objective has been to develop more convenient storage methods to hold more data and make the data easier to access. Present technologies include: microfilm, magnetic tape, floppy disks, CD-ROM, and write-once read-many (WORM) optical disks.
A major problem with the technologies available is that they use mechanical heads for reading, writing, and erasing which touch the recording material. A sudden vibration or dust particles cause the mechanical heads to physically bump into the recording material, which can damage it. These damaging events reduce the life and reliability of tapes and floppy disks. Although WORM disks are optical disks, information recorded on a WORM disk can neither be erased nor rerecorded. Another problem is that present disks are limited in the amount of storage space available.
Erasable magneto-optical disks have recently been added to the above technologies. See, for example, "Optical Disks Become Erasable," IEEE Spectrum, Vol. 25, 41-45 (Feb. 1988). The erasable magneto-optical disk is the only member of the present technology thus far, to provide for erasable optical storage. Although erasable magneto-optical disks are faster than the previous technology, problems associated with their use exist. For example, information may be erased or altered by certain magnetic fields within the proximity of the disk. Further, erasable magneto-optical disks may experience domain-wall migration, which is the tendency of magnetic patterns to bleed into adjacent areas. Also, erasable magneto-optical disks have limited write-erase cycle lifetimes of about 10.sup.4 cycles.
A photoferroelectric optical information storage device which uses the field-induced AFE-to-FE phase transition in AFE-phase PLZT compositions to store high-resolution, high-contrast optical information was described in "Photoferroelectric Image Storage in Antiferroelectric-Phase PLZT Ceramics," by Cecil E. Land, IEEE Trans. on Electron Device, Vol. ED-26, No. 8, 1143-1147 (1979). In this device, electric-field-induced AFE-to-FE phase transition is photoinhibited by exposure to near-UV light with photon energies equal to or greater than the band gap (about 3.4eV). This storage device employs an intrInsIc photoferroelectric effect similar to that occurring in FE-phase compositions. This photoferroelectric effect in the AFE-phase material provides a basis for erasable optical information storage such as photographic images with high resolution and contrast or holograms with large diffraction efficiencies. In unimplanted AFE-phase PLZT, the intrinsic photoferroelectric effect relies on photon absorption to photoexcite carriers into the conduction band and thereby to increase the threshold voltage for field-inducing the AFE-to-FE phase transition.
Both resolution and contrast of stored photographic images and the diffraction efficiency of stored holograms are substantially higher for storage in AFE-phase material than for comparable photoferroelectric storage in FE-phase compositions. The absence of domain structure in the AFE-phase removes a major source of light scattering and thereby increases contrast and/or diffraction efficiency of stored information.